The invention relates to a hydraulic device for controlling the pressure medium flow to and/or from a single-acting cylinder subjected to a load, with a pump that delivers the pressure medium from a tank to the single-acting cylinder, with a valve array located between the pump and the single-acting cylinder, said array, together with a switching valve that returns the pressure medium to the tank in one switch position, controlling the pressure medium flow to and/or from the single-acting cylinder, especially for controlling the lifting mechanism of a mobile machine.
A device of this kind for controlling the pressure medium flow to and/or from a single-acting cylinder subjected to a load is known from DE 40 30 952 Al. No details of the load that is applied to the single-acting cylinder are provided in this patent. An electrically actuated shutoff valve is located between a fixed displacement pump and a singe-acting cylinder. This shutoff valve forms a valve array with two connections that operates as a check valve in one flow direction of the pressure medium and allows a continuous fine control of the pressure medium flow in the other flow direction. The shutoff valve and an additional switching valve that is likewise electrically actuated control the pressure medium flow to and/or from the cylinder. A switching valve in a first switch position connects the fixed displacement pump with the shutoff valve. In the other switch position of the switching valve, it connects the fixed displacement pump with the tank as well. In the first switch position of the switching valve, pressure medium flows from the fixed displacement pump through the valve array which acts as a check valve in this flow direction. The connection from the fixed displacement pump to the tank is thus blocked. The piston of the cylinder is extended at maximum speed, corresponding to the volume flow of the fixed displacement pump and the dimensions of the cylinder, and the load to which the cylinder is subjected is raised. In the second switch position of the switching valve, pressure medium supplied by the fixed displacement pump flows back directly to the tank. As the load falls, pressure medium forced out of the cylinder is also returned to the tank. If the shutoff valve is not actuated, the shutoff valve closes. Then no pressure medium flows out of the cylinder and the load is held. If the shutoff valve and the switching valve are both actuated, pressure medium flows out of the cylinder through the shutoff valve and the switching valve to the tank. Fine control of the pressure medium flow returning to the tank through the shutoff valve is provided by the level of the electrical control signal supplied to the shutoff valve. When lifting the load, in contrast to lowering of the load, it is not possible to control the speed.
To control the lifting mechanism of a mobile machine, for example the lifting mechanism of a forklift or an agricultural machine, constantly operating proportional valves are generally employed to which pressure medium is supplied by a fixed displacement pump. These valves permit a continuous change in the speed at which the lifting mechanism is raised or lowered. It has been found in practice that in many instances, for example in plows or the cutters of combines, a continuous change in the adjustment rate of the lifting mechanism is not necessary, but two different speeds for raising and lowering are sufficient for adjusting the lifting mechanism, a first speed with which considerable positioning travel can be rapidly achieved and a second speed slower than the first speed for slowly lifting and lowering the tool mounted on the lifting mechanism for fine positioning, for example when coupling to the lifting mechanism or uncoupling from the lifting mechanism. A control of this kind can be provided more economically with switching valves than with proportional valves. The device operates at the fast speed until it comes into the vicinity of the desired position and then a fine positioning is performed at the slow speed. The fixed displacement pump is designed so that it delivers the pressure medium flow required for rapidly lifting the load. When the load is lifted rapidly, the pressure medium flow supplied to the single-acting cylinder (consumer flow) is equal to the pressure medium flow (pump flow) delivered by the fixed displacement pump. When lifting the load slowly, the consumer flow is smaller than the pump flow in accordance with the lower adjusting speed. The reduction in the consumer flow required for lifting the load slowly is provided by a suitably dimensioned throttle in the valve array located between the pump and the consumer. The difference between the pump flow and the consumer flow in the simplest case is returned through a pressure-limiting valve to the tank. The pump pressure rises until it reaches the limiting pressure of the pressure-limiting valve. Since the pressure-limiting valve serves primarily as a safety valve, the limiting pressure that is set is higher than the maximum load pressure that develops during operation. The pump pressure thus rises to a greater value when lifting slowly than when lifting rapidly, when the pump pressure is only slightly above the load pressure. The consumer flow during slow lifting is determined by the cross-sectional area of the throttle and the pressure drop at the throttle. The pressure drop at the throttle is equal to the difference between the limiting pressure of the pressure-limiting valve and the load pressure. Since the limiting pressure of the pressure-limiting valve, once set, remains constant, the consumer flow depends only on the size of the load. As the load pressure rises, the pressure differential between the limiting pressure and the load pressure falls and the consumer flow decreases with the root of the pressure differential. This means that when lifting slowly, small loads are raised more rapidly than large loads.